Composite structures, such as wind turbine blades, usually comprise a plurality of layers of sheets made of fibres embedded in a resin. These semi-finished component can be called pre-forms.
Traditionally, such preforms have been produced by cutting layers of sheets from an endless sheet material of fibres in a resin, and stacking them in layers on each other to form a composite structure. The composite structure is then consolidated by removal of any air trapped between the layers of the structure, usually by applying a vacuum. During this consolidation procedure, additional resin may be introduced to, or infused in, the structure to fill any voids therein to strengthen the structure.
Air trapped in the pre-form may decrease the strength of the finished pre-form, and is therefore unwanted.
WO 2009/077582 discloses a method for preparing a pre-form comprising several layers of fibre tows being fixed by a resin. In order to remove trapped air from the pre-form, an encapsulation step is suggested, allowing air removal from the enclosure to draw out the trapped air. To facilitate removal of trapped air, the layers of resin may be distributed in a non-continuous manner.
WO2004/078442 discloses a method for fabricating a pre-consolidated pre-form comprising a resin and a plurality of layers of oriented fibers arranged in fibre tows.
A pre-form is a composite material comprising fibres and an uncured or only partly cured resin. The fibres are preferably provided in layers of oriented fibres like for example individual fibres, fibre tows, fibre tow-pregs or prepregs.
By pre-consolidation is herein meant a process, whereby gas inside a pre-form is removed and a low porosity pre-form is produced. Pre-consolidation involves redistribution of a resin and optionally a redistribution of fibres. Furthermore, pre-consolidation may involve a limited curing of the resin. Pre-consolidation is particularly useful as it produces a dense pre-form (hereinafter named a pre-consolidated pre-form). Pre-consolidated pre-forms and composites prepared from pre-consolidated pre-forms will be appreciated amongst others due to good reproducibility, low porosity, high homogeneity, high strength, ability to plastically shaping of the pre-consolidated pre-form, ability to be connected to other pre-forms and/or other structures, suitability for automation and long shelf life without premature curing.
The present invention provides an improved technique of facilitating the removal of trapped air in a pre-consolidating process. The present invention provides an improved technique for reducing the occurrence of intra-layer voids during the formation of a pre-consolidated prepreg. With intra-layer voids is here intended voids that may appear inside a prepreg structure.